Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Fri Oct 19 2018 Valid 12Z Mon Oct 22 2018 - 12Z Fri Oct 26 2018 ...Guidance/Predictability Assessment and Weather Highlights/Threats... Split flow will initially set up across North America with a weak, stretched out southern stream anchoring the Desert Southwest. This feature should gradually press eastward in response to a lowering of upstream heights. Across the higher latitudes, highly amplified flow will be in place with a mean upper low spinning over the upper reaches of Nunavut. A pair of shortwaves embedded in this flow are primed to intensify and eventually become negatively-tilted over the vicinity of New England. Many solutions show a rather deep surface cyclone over the Canadian Maritimes by mid-week. Back to the northeastern Pacific, a very active pattern sets up consisting of at least a pair of powerful surface lows crashing into coastal British Columbia. Some solutions deepen such waves below 965-mb in some cases while shearing into the downstream ridge. Ultimately the pattern favors the eastward migration of a mean ridge toward central Canada while the synoptic-scale flow flattens across the northeastern Pacific. Several perturbations in the flow are primed to intensify in this regime with uncertainty looming with each instance. Looking to the Gulf of Mexico, a lingering frontal zone and eventual wave of low pressure should impact the Gulf Coast with a threat for heavy rainfall. Models have a reasonable grasp on the pattern at hand through roughly Day 4/Tuesday before uncertainty looms on both coasts. The 12Z UKMET remained south of the consensus off the New England Coast by mid-week with 12Z ECMWF ensemble spread improving relative to yesterday. Across the Pacific, the conveyor belt of shortwaves traversing the active flow will remain an issue given the depth of many such features. Decent clustering exists with the initial wave off the British Columbia coast on Tuesday with more uncertainty looming offshore with the next amplifying shortwave. Run-to-run spatial differences are noted with GFS solutions with this issue noted among other global models. At the very least, ensemble means are juxtaposed atop one another which will be the preferred route late in the period (Day 6/7, October 25/26). Before then, a multi-model blend of the 18Z GFS and 12Z ECMWF/UKMET seemed reasonable early on. A heavy rainfall threat will remain along the western/central Gulf Coasts with the axis moving eastward in time accompanying the weak area of low pressure. It remains to be seen how much instability will approach the coast but hefty QPF is noted in the guidance just offshore. The forecast itself tends to keep this axis right along the coast but the ultimate placement with be heavily driven by where the instability sets up. Across the Pacific Northwest, semi-persistent onshore flow will keep conditions wet over the Olympics and Cascades throughout the period. Pacific dominated air masses will preclude any notable winter weather threats. Across New England, some snow may fall over northern Maine if the low pressure were to deepen enough to pull in colder thicknesses down from eastern Canada. Regarding temperatures, below average numbers will prevail over much of the central/eastern U.S. During the early to middle part of next week, readings could be around 10 to 15 degrees below climatology over the Southern Plains with persistent northerly flow and active precipitation. Over the northwestern U.S., air masses of Pacific origin will keep mild air in place through at least the middle of next week. Rubin-Oster WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml